The outline of a known method for producing a solid fuel by using porous coal as the raw material is explained. Porous coal (raw coal) is pulverized at a pulverization step and thereafter mixed with mixed oil containing a heavy oil content and a solvent oil content at a mixing step, and thereby a raw slurry is obtained. Successively, the raw slurry is preheated and thereafter heated at an evaporation step, thereby the porous coal is dewatered and the mixed oil is impregnated into the pores of the porous coal, and thereby a dewatered slurry is obtained. Thereafter the upgraded porous coal and the mixed oil are separated from each other in the dewatered slurry at a solid-liquid separation step and thereafter the upgraded porous coal is dried at a final drying step. The dried upgraded porous coal is cooled and briquetted if desired and thus a solid fuel is obtained. Meanwhile, the mixed oil recovered at the solid-liquid separation step and the final drying step is recirculated to the mixing step for obtaining a raw slurry and reused as recycle oil.
An example of the schematic configuration diagram of an apparatus adopting such a method is shown in FIG. 4 (Patent Document 1). The apparatus shown in FIG. 4 comprises a mixing tank 101 for mixing porous coal with mixed oil containing a heavy oil content and a solvent oil content and thereby producing a raw slurry, an evaporator 102 for processing evaporation of water from the raw slurry, and a solid-liquid separator 103 for separating solid and liquid from each other in the dewatered slurry having been processed for the evaporation of water. Here, the mixing tank 101 includes slurry circulating flow channels 111 and 113 for introducing the raw slurry from the lower part of the mixing tank 101 to the upper part thereof with a slurry pump 112. The evaporator 102 includes slurry circulating flow channels 121, 123, and 124 for introducing the slurry from the lower part of the evaporator 102 to the upper part thereof with a slurry pump 122. A raw slurry supplying flow channel 114 is installed between the mixing tank 101 and the evaporator 102. The raw slurry supplying flow channel 114 branches from the slurry circulating flow channels 111 and 113.
A raw slurry is produced by mixing porous coal with mixed oil containing a heavy oil content and a solvent oil content in the mixing tank 101. The raw slurry: is introduced from the lower part of the mixing tank 101 to the upper part thereof through the slurry circulating flow channels 111 and 113 with the slurry pump 112; and is circulated. With being circulated, the raw slurry: enters the slurry circulating flow channels 123 and 124 of the evaporator 102 through the raw slurry supplying flow channel 114 branching from the slurry circulating flow channels 111 and 113; and enters the evaporator 102 therethrough. On this occasion, the raw slurry is heated with heat exchangers 110a and 115a (a preheating step), is further heated with a heat exchanger 120, and enters the evaporator 102 (an evaporation step). At the evaporator 102, water in the raw slurry is evaporated.
The dewatered slurry obtained by the water evaporation enters the solid-liquid separator 103 for separating solid and liquid from each other through a dewatered slurry supplying flow channel 125 branching from the slurry circulating flow channels 121, 123, and 124 and thereby a solid content (upgraded porous coal) and a liquid content (mixed oil) are obtained. An oil content remaining in the solid content is recovered at a final drying section (not shown in the figure) and the solid content comes to be the state of being used as a powdered solid fuel. Meanwhile, the recovered mixed oil is returned to the mixing tank 101 by a circulation means 104.
With the apparatus shown in FIG. 4 however, when trouble occurs at the evaporation step or subsequent steps, problems arise if the supply of raw coal (RC) and mixed oil (MO) at the mixing step and the supply of slurry to the subsequent steps are stopped in order to prevent slurry from overflowing at each step. More specifically, on this occasion, slurry pumps and stirrers are always operated in order to avoid the clogging of pipes caused by the sedimentation of coal in the slurry at the mixing step and the evaporation step but the clogging of pipes occurs in a heat exchanger as it will be shown below.
Many pipes through which slurry passes are installed in a heat exchanger as shown in FIG. 5(A) and the slurry in the individual pipes is heated by a heating medium such as steam. In the apparatus shown in FIG. 4, when the supply of raw coal (RC) and mixed oil (MO) and the supply of slurry to subsequent steps are stopped and the slurry pumps 112 and 122 are always operated, the slurry stops to flow and coal in the slurry settles out in the heat exchangers 110a and 115a and the coal (RC) at the lower part and the mixed oil (MO) at the upper part separate from each other in each of the heat exchangers as shown in FIG. 5(B). The coal settled at the lower part deposits firmly and hence it is necessary to feed gas or slurry of a high pressure and pressurize the clogged pipes in order to feed the slurry again. If the pipes in a heat exchanger are pressurized however, a pipe of a relatively low degree of clogging is unclogged firstly, a pipe of a relatively high degree of clogging cannot be pressurized effectively, and the pipes are not completely unclogged.
In view of the above situation, Patent Document 2 reports a technology of installing heat exchangers 110b and 115b in slurry circulating flow channels 111 and 113 of a mixing tank 101 as shown in FIG. 6 in place of the heat exchangers 110a and 115a in FIG. 4. In such a configuration, the flow of slurry is maintained in the heat exchangers 110b and 115b by a slurry pump 112 and hence the clogging of pipes caused by the sedimentation of coal does not occur. The reference symbols in FIG. 6 represent the same instruments and members as those represented by the same reference symbols used in FIG. 4 respectively and hence the explanations thereof are omitted.
Patent Document 1: JP-A No. 233383/1995 (H7)
Patent Document 2: JP-A No. 206695/2005